1. Field of the Invention
The present invention relates to a detection technique of identifying a defective recording element among a plurality of recording elements in a recording head (for example, an inkjet head) by means of a result of test pattern recording by the recording head, and an image forming technique to which the detection technique is applied.
2. Description of the Related Art
Methods of recording an image on a recording medium such as a sheet of recording paper (hereinafter referred to as a “recording sheet”) include an inkjet image forming system in which ink droplets are ejected from a recording head in accordance with an image signal and the ejected ink droplets are deposited onto the recording medium. An example of an image forming apparatus using such an inkjet image forming system is a full-line head image forming apparatus in which an ejection unit (including a plurality of nozzles) that ejects ink droplets is linearly disposed correspondingly to an entire range of one side of a recording medium, and the recording medium is conveyed in a direction perpendicular to the ejection unit in order to enable an image to be recorded on an entire area of the recording medium. Since the full-line head image forming apparatus is capable of forming an image on an entire area of a recording medium by conveying the recording medium without moving the ejection unit, the full-line head image forming apparatus is suitable for increasing recording speed.
However, with a full-line head image forming apparatus, an actual dot position that is actually recorded on a recording medium can be rendered deviated from an ideal dot position due to various reasons such as production tolerance, deterioration with age, or the like of recording elements (including nozzles) constituting the ejection unit, and the deviation causes a recording position error (droplet deposition position error). As a result, a problem arises in that streaky artifacts occur in an image recorded on the recording medium. In addition to artifacts due to such a recording position error, there are phenomena in which streaky artifacts occur in an image recorded on the recording medium due to failures in a recording element such as an abnormality in which droplets are not ejected (non-ejection), an abnormality in ejection volume, and an abnormality in ejection shape (e.g., splash). Such recording elements which cause a decline in recording quality are collectively referred to as “defective nozzles” or “defective recording elements”.
Since a length of a full-line recording head is equivalent to a width of a recording sheet, for example, when recording resolution is 1200 DPI (dots per inch), recording elements of an apparatus capable of accommodating a recording sheet having a width similar to that of half Kiku size (636 mm by 469 mm) number approximately 30,000 nozzles per ink. With such a large number of recording elements, defective nozzles can occur at various timings. More specifically, a nozzle can become defective at the time of manufacture of a recording head, a nozzle can become defective due to deterioration with age, a nozzle can become defective during maintenance (when a nozzle suffers maintenance-induced defectiveness, the nozzle is often restored by a next maintenance), and a nozzle can become defective midway through continuous printing.
A technique is known in which, when a defective nozzle occurs, usage of the defective nozzle is suspended (ejection suspension) and other surrounding nozzles capable of normal ejection are used in order to correct an image. When applying such a correction technique, it is important to accurately identify the defective nozzle.
As techniques for identifying defective nozzles, Japanese Patent Application Publication Nos. 2007-054970, 2000-221424 and 05-301426 describe methods of identifying defective nozzles by printing predetermined test patterns aimed at detection of defective nozzles, reading a printing result with an image reading device, and analyzing read image data obtained.
Japanese Patent Application Publication No. 2007-054970 describes a technique involving using a scanner that reads at a lower resolution than a resolution of a recording head and interpolating with respect to read data to detect a defective nozzle. However, the publication does not make any mention of distortion of a read image and a problem and a solution of image distortion. Moreover, with the technique described in the publication, a sufficiently accurate detection result cannot be obtained under a condition where a width of a line formed by dots on a test pattern does not satisfy a sampling theorem, since a certain amount of error (an estimation error of a line profile formed by dots) remains on an identified line position.
Japanese Patent Application Publication No. 2000-221424 describes a technique in which an image fluctuation correction pattern is disposed additionally and adjacently to a detection pattern in order to correct a read image fluctuation. However, disposing the correction pattern additionally to the detection pattern as described in the publication increases used area in the recording sheet and problematically increases area in the recording sheet that is worthless from a user's perspective. Moreover, the publication does not make any mention of an issue as to whether or not distortion of a read image necessitates correction of the image structure.
Japanese Patent Application Publication No. 05-301426 describes in paragraph 0037 that an output of a sensor is subjected to output correction or so-called shading correction by assuming that a recording material prior to pattern printing is a white level, but does not make any mention of a technique for acquiring shading correction data from a read image of a pattern to perform correction.
<Problems in Analyzing a Read Image>
When reading a test pattern printing result with an image reading device such as a scanner and analyzing the read image data, a distorted image of the test pattern can sometimes be read during acquisition of the read image data.
For example, the following phenomena (1) to (3) occur in an inkjet printing apparatus adopting a conveyer system in which a leading end portion of a recording sheet is gripped with grippers (gripping claws) and the recording sheet is fixed to a peripheral surface of a drum (cylinder) and conveyed by a rotation of the drum, and has an image reading sensor disposed at a position opposing the drum for reading a printing result.
(1) Since the leading end portion of a recording sheet is pinned by the gripper, an amount of sinking of the recording sheet differs depending on how the recording sheet is pressed down and also on a rigidity of the recording sheet, and a distance between the recording sheet and the image reading sensor fluctuates according to a position on a recording sheet surface. The distance fluctuation renders distortion of a read image.
(2) A vibration of the apparatus or the like can cause an image reading optical system to sway and can generate swaying or winding of a line on the read image which reflects the vibration.
In the present specification, distortion of read images occurring due to various factors as exemplified in (1) and (2) above is collectively referred to as “distortion”. Such distortion of read images lowers a detection accuracy of defective nozzles.
(3) A reading device including an image reading sensor is provided with an illuminating light source that irradiates light to a read position on a recording sheet. A difference in an amount of sinking of the recording sheet causes an uneven light distribution of the illuminating light and, depending on a position on a recording sheet surface, illuminating light intensity fluctuates. Moreover, differences in light intensity can occur due to a fluctuation in illuminating light intensity of the light source and due to light distribution of the light source.
The above-described phenomena (1) to (3) are irreproducible, and circumstances such as how a recording sheet is gripped and a state of a light source differ for each recording sheet and for each occurrence. Therefore, such irreproducible distortion and light intensity fluctuation (shading) are fluctuation factors that are difficult to correct using a fixed correction table or the like.